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  Java Practices -> Construct classes from the outside in
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<div class='page-title'>Construct classes from the outside in</div>

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<br>When experimenting with the design of a new class, it is very helpful
to approach the task from the point of view of the <i>caller</i> :
<ul>
<li>
the quality of the final design will be higher, since there is more feedback
from use cases</li>

<li>
the class is often constructed <i>significantly</i> faster, since discovery
of poor design decisions usually happens much earlier</li>
</ul>
Much of design is summed up simply as :
<ul>
<li>
make your best guess</li>

<li>
find and fix the errors</li>

<li>
repeat</li>
</ul>
The critical step seems to be the second. <i>The best techniques are those
which discover the errors the fastest</i>. Coding "from the outside in"
seems to perform very well in this regard.

<p>Constructing a non-private method from the outside in might involve
these steps :
<ul>
<li>
write some code from the point of view of the caller</li>

<li>
<i>make the code compile by providing a simple stub implementation of the
method you are designing</i>. For a <tt>void</tt> method, such a stub is
simply an empty method. For a method which has a return value, such a stub
might simply return <tt>null</tt>, <tt>0</tt>, <tt>false</tt>, or an empty
<tt>String</tt>.</li>

<li>
experimenting with different designs is simple at this point, since
the amount of calling code is minimal, and the implementations are toys</li>

<li>
correctness of method name and return type are determined by the needs
of the caller</li>

<li>
correctness of arguments seems to be determined more by the needs of the
implementation, not the caller</li>

<li>
<i>is the return type correct</i>? This is an important item, since changes
in return type will often have large ripple effects in the caller.</li>

<li>
is the method intelligible at the point of call?</li>

<li>
when the design is more or less stable, write the javadoc of the method as
the formal specification of the <a href="TopicAction63b6-2.html">contract</a> between
the method and its callers. Often, this will also help clarify details
and border cases.</li>

<li>
finally, replace the stub implementation with a real implementation, and
<a href="TopicActiond5a5-2.html">test</a> it</li>
</ul>
<span class='highlight'>In effect, such techniques use the programming language itself as a design tool.</span>
<br>
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<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicActiond5a5-2.html?Id=33'>Use a testing framework (JUnit)</a> <br>
 
  
  <a href='TopicActionebc1-2.html?Id=60'>Use javadoc liberally</a> <br>
 
  
  <a href='TopicAction311e-2.html?Id=136'>Separate public and private members</a> <br>
 
  
  <a href='TopicAction63b6-2.html?Id=194'>Design by Contract</a> <br>
 
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